The treatment of products using radiation is well established as an effective method of treating materials such as medical devices or food stuffs. Radiation processing of products typically involves loading products into totes and introducing a plurality of totes either on a continuous conveyer, or in bulk, into a radiation chamber. Within the chamber the product stacks pass by a radiation source until the desired radiation dosage is received by the product and the totes are removed from the chamber. As a plurality of products, typically within totes, are present in the chamber at a given time, the radiation processing parameters affect all of the product within the chamber at the same time.
One common problem in the radiation processing of products is that the effectiveness of radiation processing is sensitive to variations in product density and geometry, and product source geometry. If a radiation chamber is loaded with totes comprising products with a range of densities and geometries,certain products will tend to be over-exposed to the radiation, while others do not achieved the required dose, especially within the central regions of the product. To overcome this problem the radiation chamber is typically loaded with products according to a specified and validated configuration so that the processing of the products satisfies a specified dose uniformity criteria. However, this is not always possible as some product package configurations are not compatible with achieving a good dose uniformity when irradiation is carried out in the conventional manner.
Products of a large dimension, and high density suffer from a high dose uniformity ratio (DUR) across the product. A relatively even radiation dose distribution (small DUR) is desirable for all products, but especially so for the treatment of foods, such as red meats and poultry. In treatment of these products, an application of an effective radiation dose to reduce pathogens at the centre of the stack is often limited by associated undesirable sensory or other changes in the periphery of the product stack as a result of the higher radiation dose delivered to material in this region of the product. A similar situation may arise during the radiation sterilization of medical disposable products, a majority of which may be made from plastic materials. In these cases, the maximum permissible radiation dose in a product may be limited by undesirable changes in the characteristics of the plastics, such as increased embrittlement of polypropylene or decoloration and smell development of polyvinyl chloride. In order to adequately and thoroughly treat product stacks of such products with radiation processing, a relatively even radiation dose distribution characterized by a low DUR must be delivered throughout the product stack.
Radiation processing of materials and products has most often been accomplished using electron beams, gamma radiation or X-rays. A major drawback to electron beam processing, is that the electron beam is only capable of penetrating relatively shallow depths (i.e. cm) into product, especially high density products such as food stuffs. This limitation reduces the effectiveness of electron beam processing of bulk or palletized materials of high density. Gamma radiation is more effective in penetrating products, especially those of a higher density or larger dimensions, compared with electron beam. Most gamma sources are based on radioactive nuclides such as cobalt-60. Kock and Eisenhower (National Research Council of the National Academy of Sciences Publication #1273; 1965) discuss the merits of different types of radiation processing for the purposes of food treatment. The article suggests that photons are the preferred source for treating large product stacks because of the greater ability of photons to penetrate the product.
U.S. Pat. No. 4,845,732 discloses an apparatus and process for producing bremsstrahlung (X-rays) for a variety of industrial applications including irradiation of food or industrial products. An alternate device for the production of X-rays is disclosed in U.S. Pat. No. 5,461,656 which also discloses X-ray irradiation of a range of materials. U.S. Pat. No. 5,838,760 and U.S. Pat. No. 4,484,341 teach a method and apparatus for selectively irradiating materials such as foodstuffs with electrons or X-rays. None of these documents discloses an apparatus or methods to deliver a relatively even radiation dose distribution, especially in large product stacks of high density, so that a low DUR is achieved in treated products.
U.S. Pat. No. 4,561,358 discloses an apparatus for conveying articles within a tote (carrier) through an electron beam. The invention teaches of a carrier that is capable of reorienting its position as the carrier approaches the electron beam. An analogous system is disclosed in U.S. Pat. No. 5,396,074 wherein articles are transported past an electron beam on a process conveyor system. The conveyor system provides for re-orientation of the carrier so that a second side (opposite the first side) of the carrier is exposed to the radiation source. The carrier is further defined in U.S. Pat. No. 5,590,602. A similar electron beam irradiation device is disclosed in U.S. Pat. No. 5,994,706. An apparatus to optimize the dosage of electron beam radiation within a product are given in U.S. Pat. No. 4,983,849. The apparatus includes placing cylindrical or plate dose attenuators between the radiation beam and product. The attenuators comprise a moving, perforated metal plate (or cylinder) scatter the radiation beam and reflect non-intersecting electrons thereby increasing dosage uniformity.
U.S. Pat. No. 5,554,856 discloses a radiation sterilizing conveyor unit for sterilizing biological products, food stuffs, or decontamination of clinical waste and microbiological products. Products are placed on a disk-shaped transporter and rotated so that the products are exposed to a field of accelerated electrons. A similar apparatus for electron beam sterilization of biological products, foodstuffs, clinical waste and microbiological products is also disclosed in U.S. Pat. No. 5,557,109. Products are placed in a recess or pocket of a manipulator which is slid horizontally into a cavity until the products are aligned with a path of an electron beam housed within the sterilization unit.
In the prior art systems described above, there are limitations in the ability to deliver a relatively flat dose distribution (low DUR) throughout a product or product stack since no method is provided to compensate for the different doses received by the exterior and interior portions of the product stack. This therefore results in the outer portions of a product to receive a much higher radiation dose than that received within the product stack.
U.S. Pat. No. 4,029,967 and U.S. Pat. No. 4,066,907 disclose an irradiation device for the uniform irradiation of goods by means of electro-magnetic radiation having a quantum energy larger than 5 KeV. Products to be irradiated (including medical articles, feedstuffs, and food) rotate on turntables and are partially shielded from a radiation source by shielding elements. There is no discussion of optimizing the geometry of the radiation beam relative to the product stack, or modifying the spacing of the shielding elements in order to optimize the DUR within a product. As a result, products with different densities are still subject to a wide range in DUR as is the case with other prior art systems. U.S. Pat. No. 5,001,352, also discloses a similar apparatus comprising product stacks that rotate on turntables, positioned around a centrally disposed radiation source, and shielding elements that reduce lateral radiation emitting from the source. A shielding element comprising a plurality of pipes that are fluid filled thereby permitting flexibility in the form of the shielding element is also discussed. However, there is no guidance as to how this or the other shielding elements are to be positioned in order to attenuate the radiation beam relative to the product stack in order to optimize the DUR within the product. Nor is there any discussion of any real-time adjustment of shielding elements to optimize the dose distribution received by a product that accounts for alterations in product densities.
A major limitation with the prior art irradiation systems is that it is difficult to obtain a relatively even radiation dose distribution (low DUR) throughout a product or product stack. For example, in systems which irradiate products from only one side, the material irradiated at the periphery of the product and closest to the irradiation source receives a high radiation dose relative to the product located at the center regions of the product stack, and further away from the radiation source resulting in a high DUR. Even with systems that irradiate products from multiple sides, the material irradiated at the periphery of the product typically receives a higher dose of radiation than the material located at the centre of the product since the radiation method is not optimized for the product stacks. Consequently, the product receives an uneven dose of radiation, characterised by a high DUR. Thus, prior art systems are limited in their ability to deliver a relatively flat dose distribution (low DUR) throughout a product or product stack. These limitations are more pronounced in larger products, with higher densities.
It is an object of the current invention to overcome drawbacks in the prior art.
The above object is met by the combinations of features of the main claims, the sub-claims disclose further advantageous embodiments of the invention.